The present invention relates to land or air mobile communication which adopts a multi-channel access system such as a time division multiple access (TDMA) system or a frequency division multiple access (FDMA) system which employs a frame configuration including an idle time which is not used for communication of the local station. More particularly, the invention pertains to radio communication equipment adapted to prevent a momentary interruption of communication by hand-off during communication and a traffic channel hand-off method utilizing such radio communication equipment.
With mobile radio communication equipment employing the multi-channel access system, communication is performed over a channel selected from a plurality of radio channels. To this end, a local oscillator is designed to output one of a plurality of frequencies and is often formed by a PLL frequency synthesizer. FIG. 1 shows an example of the construction of conventional multi-channel access radio communication equipment. A transmitting/receiving antenna 11 is connected via a duplexer 12 to a transmitting part 13 and a receiving part 14. The oscillation output of a local oscillator 15 is provided to the transmitting part 13 and the receiving part 14. A control part 16 controls the connection of the transmitting part 13 and the receiving part 14 to a speaker/microphone unit 17 and effects other various control for transmission and reception. When the oscillation frequency of the local oscillator 15 is changed by a frequency assigning signal from the control part 16, transmission and reception frequencies are also changed, that is, the traffic channel is handed off.
In mobile communication, when a mobile station MS moves from a zone Z.sub.1 of a base station 24 with which it is currently in communication to a zone Z.sub.2 of another base station 25 as shown in FIG. 2, the radio channel frequency f.sub.1 of the current base station 24 must be handed off to the radio channel frequency f.sub.2 of the new base station 25 so as to continue communication. The hand-off takes place in such a procedure as depicted in FIG. 3. As the mobile station MS in communication with the base station 24 through the channel of the frequency f.sub.1 approaches the boundary between the zones Z.sub.1 and Z.sub.2, the current base station 24 sends a channel assigning signal CAS to the mobile station MS to assign a new channel for communication with the neighboring base station, that is, the frequency f.sub.2 and the corresponding slot number. In the mobile station MS, upon receipt of the channel assigning signal CAS, the control part 16 of radio communication equipment 10 shown in FIG. 1 changes the oscillation frequency of the local oscillator 15 formed by a frequency synthesizer. As a result of this, the transmitting part 13 and the receiving part 14 are set to the frequency f.sub.2 of the new channel and transmission and reception are initiated on the new channel. In the communication with the same base station, the transmission from the base station to the mobile station (i.e. down link transmission) and the transmission from the mobile station to the base station (i.e. up link transmission) may be effected either at the same frequency or at different frequencies; in either case, the frequency or frequencies for transmission and reception on the same channel will hereinafter be indicated by a common reference character. Hence, the frequency of the current channel used for communication with the current base station 24 and the frequency of the new channel to be used for communication with the new base station will be identified by f.sub.1 and f.sub.2, respectively, as referred to above in respect of FIGS. 2 and 3.
In the case where the mobile communication system used employs a digital signal, it is necessary, for receiving a signal from the new base station 25, that the clock and frame of the receiving part 14 of the mobile station MS be synchronized with the signal received on the new channel of the frequency f.sub.2 and that a receiving device (not shown) of the new base station 25 also be synchronized, in clock, with the signal received from the mobile station MS. To establish such synchronization, as shown in FIG. 3, the new base station 25 transmits a down link synchronizing signal DSYN and the mobile station MS transmits an up link synchronizing signal USYN, after which the new base station 25 transmits a down link continuity test pattern signal DTP composed of a predetermined bit string so as to make sure that transmission and reception over the new channel are effected normally, and then the mobile station MS determines if the down link continuity test pattern signal DTP received is error-free (a down link continuity test). Next, the mobile station MS transmits a similar up link continuity test pattern signal UTP and the new base station 25 determines if the pattern signal UTP received is error-free (an up link continuity test). Where the requirements are fulfilled, communication is enabled over the new channel. Consequently, communication is interrupted momentarily for a period of time Tb from the time when processing for switching from the old channel to the time when communication over the new channel is enabled. It is desirable, from the viewpoint of service to users, that the time of momentary interruption be minimized, but the hand-off time in the prior art is substantially in the range of between tens of and hundreds of milliseconds, since the local oscillator 15 of the radio communication equipment 10 of the mobile station MS is usually formed by a PLL synthesizer of a low loop gain. In consequence, during the hand-off period communication is interrupted in the case of voice communication and information during that time drops out in the case of facsimile or similar data transmission. Moreover, the digital transmission system requires, after the hand-off, the time for establishing signal synchronization between the mobile station and the new base station--this inevitably further lengthens the communication-OFF period.
A hand-off method which does not incur such a momentary interruption of communication is proposed in Japanese Patent Application Laid Open No. 157533/88. According to the proposed method, the mobile station MS has two sets of transmitter-receivers (13a, 14a) and (13b, 14b) as shown in FIG. 4, and the hand-off takes place following the procedure shown in FIG. 5 as described below. When the mobile station MS communicating with the base station of the zone Z.sub.1 through the channel of the frequency f.sub.1 by use of the one set of transmitter-receiver (13a, 14a) approaches the zone Z.sub.2, the base station 24 transmits the channel assigning signal CAS to the mobile station MS to instruct it to use the frequency f.sub.2 of the zone Z.sub.2. Upon receipt of the channel assigning signal CAS, the mobile station MS sets the other set of transmitter-receiver (13b, 14b) to the channel of the assigned frequency f.sub.2. Next, processing for establishment of synchronization, similar to the aforementioned, is performed between the new base station 25 and the transmitter-receiver (13b, 14b) of the mobile station MS, using the down link and up link synchronizing signals USYN and DSYN. After this, a down link continuity test takes place using the down link continuity test pattern signal DTP, followed by an up link continuity test using the up link continuity test pattern signal UTP. In the course of the hand-off processing from the reception of the channel assigning signal CAS to the transmission of the up link continuity test pattern signal UTP, the mobile station MS communicates with the base station 24 over the channel of the frequency f.sub.1 through the transmitter-receiver (13a, 14a), and the channel of the frequency f.sub.1 is switched to the new channel of the frequency f.sub.2 after completion of the up link continuity test with the transmission of the up link continuity test pattern signal UTP. Hence, no momentary interruption will occur in the communication.
The above-mentioned hand-off method is free from the momentary interruption of communication but calls for loading the two sets of transmitters-receivers (13a, 14a) and 13b, 14b) in the mobile station MS--this is not preferable in terms of costs and the space of installation